That sounds very rattly to me. I suppose the good news is that it is a regular Vx engine as fitted to many models including, I believe, the Corsa. You should get it checked over by a Vx Ampera specialist. My advice is to take it to Bellingers - even if it means a day's drive and an overnight stay.My advice is to get it to a proper Ampera specialist. Having a second opinion would put your mind at rest and if they discover something expensive, you still have your consumer rights and can return the car to the dealer.
It definitely idles higher, I think its default 'low' speed is 1400rpm. I don't believe its an Atkinson cycle engine though, the 2nd gen Volt's are using the newer 1.5L but the 1.4 in gen1s is used in the Corsa and Astra where it is an OTTO cycle engine.It does seem to idle a bit higher than a regular engine from memory. Possibly due to it running an Atkinson Cycle?